Electrometric titrations with the gold electrode

Summary Massive gold because of the passivity of the oxide formed on its surface can be used as a substitute for platinum in electrometric titrations including acid-base and oxidation-reduction systems. The end points are attended with large jumps on the curves and coincide satisfactorily with those obtained using the hydrogen or platinum electrode or by other method.     

Bifunctional, conjugated oligomers for orthogonal self-assembly: selectivity varies from planar substrates to nanoparticles

A diphenylacetylene containing two different end groups (isonitrile and thioacetate) was synthesized, showing that the chemistry used to install each end group is compatible with that of the others. The isonitrile group binds preferentially to platinum, and the thiol group binds preferentially to gold. However, the selectivity was different when nanoparticles were compared to planar substrates.     

Self-metallization of photocatalytic porphyrin nanotubes

Porphyrin nanotubes represent a new class of nanostructures for which the molecular building blocks can be altered to control their structural and functional properties. Nanotubes containing tin(IV) porphyrins are photocatalytically active and can reduce metal ions from aqueous solution. The metal is deposited selectively onto tube surfaces, producing novel composite nanostructures that have potential applications as nanodevices. Two examples presented here are nanotubes with a continuous gold wire in the core and a gold ball at the end and nanotubes coated with platinum nanoparticles mainly on their outer surfaces. The latter are capable of photocatalytic reduction of water to hydrogen.     

Electro-oxidation of d-mannose on platinum, gold and nickel electrodes in aqueous medium

The oxidation of d-mannose is studied on platinum, gold and nickel in alkaline medium. The electro-reactivity of this compound and the rate determining step of the reactions were determined by cyclic voltammetry. Lead adatoms have important effect on the oxidation of d-mannose on platinum electrode. Five folder of increase in current densities was observed after the addition of 10⁻⁶ M Pb²⁺. Electrolyses carried out on upd-lead modified platinum, and gold electrodes show good conversion yields (75% and 80%, respectively) and reasonable selectivities towards mannonic acid. High amount of cleavage products were detected at the end of the electrolysis at nickel electrode. The reactivity-functional group relationship has been discussed considering the results of the electrolyses.     

Cyclic chronopotentiometric determination of sugars at Au and Pt microelectrodes in flowing solutions

The main advantage of the application of cyclic chronopotentiometry (CCP) in end-column CE detection arises from the fact that the detection parameters and the magnitude of the analytical signal are (in contrast with other electrochemical detection methods) independent of the ohmic polarization of the solution caused by the separation current at the detection end of the capillary. CCP was used to determine sugars on platinum and gold microelectrodes after separation by CE. The results obtained with a gold microelectrode were better. Subsequently this detection method was used for quantitative determination of sugars in honeys and for their authentication.     

Stability of Gold and Platinum Nanowires on Graphite Edges

The stability of coinage and noble metal nanowires supported on graphite steps is examined by density functional theory. In particular, we study the stability of supported gold and platinum wires and compare their chemical properties with those of surfaces and bare wires. A substantially stronger bond with graphite was found for platinum wires due to unfilled antibonding states, which are occupied in the case of gold. This difference has direct consequences for the adsorption of hydrogen. This reaction can occur either on the wire or directly on graphite steps. In the case of gold, the reaction is favoured on steps, while on platinum wires, it has no thermodynamical preferences. Our results suggest that, in early stages of wire formation, hydrogen could desorb gold from graphite, but not platinum.     

The Suitability of Various Electrode Materials for the Electrogeneration of Halogens in Constant-Current Coulometry

Abstract

During a voltammetric study of the suitability of platinum, gold, glassy carbon, tungsten, and tantalum as electrode materials for generating halogens in constant-current coulometry it was found that glassy carbon, tungsten, and tantalum are not suitable. Both platinum and gold electrodes can be used; with platinum the possibility of interference caused by formation of complexes of the electrode metal with halides is smaller than with gold.     

Nanoscale soldering of metal nanoparticles for construction of higher-order structures

We demonstrated that platinum nanoparticles dispersed in pure water were "soldered" by gold into higher-order structures such as "nanowebs". In practice, gold nanoparticles in water containing the platinum nanoparticles were melted by irradiation of a pulsed 532-nm laser, which excites selectively the surface plasmon band of the gold nanoparticles, and the melted gold nanoparticles solder the platinum nanoparticles together into the nanowebs. Optical absorption spectroscopy and electron microscopy together with electron probe microanalysis were employed to observe the structure of the nanowebs and to elucidate their formation dynamics.     

The electrochemistry of gold–platinum alloys

The electrochemical properties of gold, platinum and gold–platinum alloy electrodes under different heat treatment conditions have been studied in 0.5 M H₂SO₄ and 0.5 M NaOH. The electro-oxidation of 0.1 M ethylene glycol in 0.5 M NaOH at these electrodes has also been studied. It was found that all the gold–platinum electrodes are more active for ethylene glycol electro-oxidation than both pure gold and platinum, and that the gold–platinum electrodes in the solid solution condition are more active than the two-phase electrodes. Poisoning of all the electrodes occurs during electrolysis of ethylene glycol at a fixed potential. Potential pulsing is successful in removing the poisoning species formed at the pure gold and pure platinum electrodes. High apparent current densities are found during the first few cycles at the Au–Pt alloy electrodes. These high current densities are also associated with more severe poisoning – than at both pure gold and platinum – and longer cleaning cycles are needed to remove the poisons at these electrodes.     

Aqueous-organic phase-transfer of highly stable gold, silver, and platinum nanoparticles and new route for fabrication of gold nanofilms at the oil/water interface and on solid supports

A simple but effective aqueous-organic phase-transfer method for gold, silver, and platinum nanoparticles was developed on the basis of the decrease of the PVP's solubility in water with the temperature increase. The present method is superior in the transfer efficiency of highly stable nanoparticles to the common phase-transfer methods. The gold, silver, and platinum nanoparticles transferred to the 1-butanol phase dispersed well, especially silver and platinum particles almost kept the previous particle size. Electrochemical synthesis of gold nanoparticles in an oil-water system was achieved by controlling the reaction temperature at 80 degrees C, which provides great conveniences for collecting metal particles at the oil/water interface and especially for fabricating dense metal nanoparticle films. A technique to fabricate gold nanofilms on solid supports was also established. The shapes and sizes of gold nanoparticles as the building blocks may be controllable through changing reaction conditions.     

Preparation of polymer-protected gold/platinum bimetallic clusters and their application to visible light-induced hydrogen evolution

The dispersions of polymer-protected gold/platinum bimetallic clusters were easily and reproducibly prepared by refluxing the mixed solutions of tetrachloroaureic(III) acid and hexachloroplatinic(IV) acid in ethanol/water (1/1) at 90 ∼ 95 °C for 2 h in the presence of a protective polymer such as poly(N-vinyl-2-pyrrolidone) (PVP). The gold/platinum bimetallic clusters thus obtained were very small, well dispersed and very stable. The UV-Vis spectra and the transmission electron micrographs have indicated that each bimetallic particle has an alloy structure consisting of both gold and platinum atoms, and that the surface of the cluster particle is rich in platinum atoms and the inner core in gold atoms. The gold/platinum bimetallic clusters were used as the multi-electron redox catalysts for visible light-induced hydrogen evolution from water. The rate of hydrogen evolution depended on the mole ratio of the gold/platinum bimetallic clusters. The bimetallic clusters at the mole ratio of Au/Pt = 2/3 were the most active catalyst. The in-situ UV-Vis spectra during the reaction have indicated that the order of the aggregation in the two kinds of metal atoms is very important for structure determination of the Au/Pt bimetallic clusters. The protective polymer PVP plays a role not only in protecting hydrophobic colloidal particles in an aqueous solution, but also in determining the metal composition of the cluster surface.     

Gold-platinum bimetallic clusters for aerobic oxidation of alcohols under ambient conditions

We have developed gold/platinum alloyed bimetallic cluster catalysts supported on a cross-linked polystyrene derivative, which present much higher activity and selectivity than single metal gold or platinum clusters for aerobic oxidation of alcohols under ambient conditions.     

Studies on the extraction of platinum metals with tri-iso-octylamine from hydrochloric and hydrobromic acid: separation and determination of gold, palladium and platinum

The extraction of Pd(II), Rh(III), Ir(III), Au(III) and Pt(IV) from hydrochloric and hydrobromic acid with 5% tri-iso-octylamine solution in carbon tetrachloride has been studied. The gold extract from hydrochloric acid is yellow and absorbs at 325 nm, the palladium compound is red and absorbs at 290 nm and 467 nm, and the platinum compound is blood-red and shows absorption at 268 nm. The gold, palladium and platinum extracts from hydrobromic acid are crimson. reddish brown and blood-red, with maximum absorption at 260, 345 and 300 nm respectively. Methods have been devised for the separation of gold from platinum and for its determination and also for the simultaneous determination of palladium and platinum.     

Electrocatalysis by ad-atoms: Part IV. Enhancement of the oxidation of formic acid on Pt-Au(subs) and Au-Pt(subs) electrodes by bismuth ad-atoms

The roles of surface and bulk for electrocatalysis have been investigated. Bi ad-atoms enhance a platinum electrode to a great extent for HCOOH oxidation. In order to examine whether bulk platinum atoms are necessary for the electrocatalysis, monolayer or submonolayer amounts of platinum atoms were made to deposit on a gold electrode which is quite inactive for the oxidation. The deposition of a complete monolayer of platinum atoms on a gold electrode makes the electrode as active as a platinum electrode itself. Bi ad-atoms enhance this electrode to the same extent as they do a platinum electrode for the oxidation. Thus surface atoms, Pt and Bi atoms, having no bulk atoms on a quite inactive electrode work effectively for the electrocatalysis, the platinum atoms for the adsorption of the main reactant and the Bi atoms for blocking of the sites against the formation of poison. It is the adsorptive property of the surface that controls the electrocatalytic activity.     

Deposition of continuous platinum shells on gold nanoparticles by chemical precipitation

Continuous platinum shells consisting of ~5 atomic layers were deposited onto preformed gold seeds in aqueous medium by reducing hexachloroplatinic acid with ascorbic acid. By controlling the reduction kinetics of Pt(IV) species and the properties of the substrate, it was possible to ensure a slow and controlled deposition of platinum atoms onto the gold cores. Electrochemical evaluations revealed the presence of a compact platinum shell. The mass specific oxygen reduction activity of platinum in the AuPt core-shell nanoparticles was found to be four times higher than that of platinum black and comparable to that of polycrystalline bulk metal.     

A fire-assay and wet chemical method for the determination of palladium, platinum, gold, and silver in ores and concentrates

A method is presented for the determination of palladium, platinum, gold and silver in ores and concentrates by a fire-assay and wet chemical technique. After parting of the lead assay button with dilute nitric acid, and separation of the solution from the residue, the palladium and platinum in the solution are precipitated by the addition of stannous chloride, with tellurium as collector. The resulting precipitate is combined with the gold residue and dissolved in aqua regia, then the solution is analysed for palladium, platinum and gold by atomic-absorption spectrophotometry (AAS). Silver is determined in the original solution by AAS before the reduction step.     

Spectrophotometric determination of noble metals with tin(II) in bromide solutions

In presence of tin(II) bromide, noble metals give coloured products which are suitable for spcctrophotometric determinations. The colours are red (platinum), yellow-orange (rhodium), yellow-brown (palladium), yellow (iridium) and violet (gold) They are extracted, except for gold, with isoamyl alcohol Platinum, rhodium and palladium can be separated from irdium, and rhodium and platinum from palladium. Rhodium and platinum can be determined simultaneously.     

Gold Trace Separation from some Platinum Group Metals on Cationic Exchanger in the Presence of Amines

Abstract

The sorption behaviour of gold in the presence of amines on different kinds of cationic exchangers with phosphonic groups has bee investigated. The system Cellex P - amine has been found very useful for gold separation from platinum metals. The method was adopted for trace gold determination in PdC1₂ and platinum powder.     

Kinetics and mechanism of glucose electrooxidation on different electrode-catalysts: Part II. Effect of the nature of the electrode and the electrooxidation mechanism

A comparative study has been made of glucose electrooxidation on electrodes made of metals of group VIII, Ag, Au, Cu and glassy carbon as well as of phthalocyanines and porphyrins of cobalt, manganese and iron. It is found that considerable electrooxidation currents are observed for iridium and rhodium (group VIII), and for gold and copper (the copper subgroup). In neutral and alkaline solutions, glucose electrooxidation rates on gold considerably exceed those on platinum.Investigations have been carried out into the main regularities of glucose electrooxidation on a gold electrode in a wide range of potentials, glucose concentrations and pH values. The effect of chloride ions, gluconic acid and amino acids on glucose electrooxidation on gold have been studied.Proceeding from the direct comparison of adsorption data with polarization data obtained under the same conditions, a mechanism of glucose electrooxidation on platinum has been suggested. It is also shown that the mechanism of glucose electrooxidation on gold is similar, in many respects, to that on platinum.